The PCGN package, or Preconditioned Conjugate Gradient Solver with Improved Nonlinear Control, is one of the solvers available for MODFLOW-2005. Using the PCGN solver a specific GMS MODFLOW executable (mf2k5_..._parallel.exe) can be run with parallel processing on a single machine (though not across multiple machines on a network) greatly reducing run time.

The MODFLOW PCGN Solver dialog has the following options:

Comments (Text) – Allows up to 199 characters.

Max number of outer iterations (ITER_MO) – For nonlinear problems, this variable must be set to some number greater than one, depending on the problem size and degree of nonlinearity. If set to 1, then the solver assumes that the problem is linear and the input requirements are greatly truncated.

Max number of inner iterations (ITER_MI) – Set to some number greater than one, depending on the matrix size, degree of convergence called for, and the nature of the problem.

Head change criterion for convergence (CLOSE_R) – If set to 1 then the problem will be linear. If set to be greater than 1 then it will be a nonlinear problem.

Residual criterion for convergence (CLOSE_H) – Used as an alternate stopping criterion for the Picard iteration needed to solve a nonlinear problem.

Relaxation parameter (RELAX) – Relaxation parameter for the modified incomplete Cholesky (MIC) preconditioner. Value can be between 0 and 1. Generally a value between 0.9 and 0.99 is advised for most problems.

(1) Adaptive – Changes the damping parameter θ in response to the difficulty the nonlinear solver encounters in solving a given problem.

(2) Enhanced – The value of θ is increased (but never decreased) provided the Picard iteration is proceeding satisfactorily.

Damping restriction (DAMP) – Restricts the damping parameter θ. Generally is set to be between 0 and 1.

Damping bound (DAMP_LB) – Represents a bound placed on θ.

Rate (RATE_D) – If using adaptive damping, sets the recovery rate for the damping factor θ in response to the progress in the Picard iteration. If using enhanced damping, adjusts the damping factor θ upward.

Maximum head change (CHGLIMIT) – Limits the maximum head change applicable to the updated hydraulic heads in a Picard iteration.